Resolved issue where a FF was None.
Resolved issue where a FF was None.

The function return_ff would return None if ParamError was raised.
This could later lead to problems when trying to evaluate this FF
(because it was actually None).

Now it checks whether the object is None before appending to the
list of trial FFs.

Q2MM/q2mm,ericchansen/q2mm,Q2MM/q2mm,ericchansen/q2mm,arosale4/q2mm,arosale4/q2mm